A layered compound such as LiCoO2 or LiNi1/3Mn1/3CO1/3O2 or a spinel compound such as LiMn2O4 has been conventionally used as a positive electrode material (positive electrode active material) for a lithium ion secondary battery. Among the layered compounds, those with high Ni content such as LiNiO2 are known to exhibit high capacity. However, as described in Patent Document 1 and Patent Document 2, a lithium ion secondary battery in which a compound with high Ni content such as LiNiO2 is used has the disadvantage of low theoretical density compared with a Co-containing compound such as LiCoO2, resulting in low energy density as a battery.
The electrode density may be increased by a method such as a roll press. However, when the electrode density is increased as described in Patent Document 3 to achieve higher electrode density, high-rate discharge characteristics may deteriorate.
In the following, the lithium ion secondary battery may be referred to as a “battery”.
Patent Document 1: JP-A-2003-257434
Patent Document 2: JP-A-2004-59417
Patent Document 3: JP-A-2008-293988